Prior patent GB-A-1093714 describes a blind hole anchor nut fastener which comprises a tubular sleeve and a screw threaded nut disposed at the end of the sleeve, the arrangement being such that when a screw threaded mandrel extending through the sleeve and engaged with the nut is displaced axially in the appropriate direction, the nut shears from the sleeve and is drawn into the sleeve in wedging engagement therewith.
Effectively the sleeve is expanded by the nut, and to enable this expansion to take place the nut is generally frusto-conical with its smallest end at the shear point and of a suitable diameter to enter the sleeve.
These anchor nuts are extremely difficult to manufacture satisfactorily. The tolerances are small, because if the nut is made too small relative to the sleeve it will not wedge satisfactory, and if it is made too large the sleeve splits. If the shear point is too weak, nuts may shear as a result of handling in manufacture and transportation, and if the shear point is too strong, shearing will not take place but the sleeve will bell out and be unsatisfactory for that reason.
Satisfactory manufacture is possible, indeed it is believed that nuts of said GB-A-1093714 have been extremely successful commercially but this has only been so because of great care in manufacturing within the prescribed tolerances and with great difficulties in selecting material.
Prior patent EP-071407 solves the above mentioned problem, providing a nut fastener device for a blind hole anchor of the same kind as in the mentioned GB-A-1093714, but having at least one back tapered face which is arcuate about an axis eccentric to that of the sleeve, and provided on the nut. In the preferred arrangement there are a plurality of back tapered faces each arcuate about a separate eccentric axis.
However, certain problems remain even with improved device of the mentioned EP, in particular the angular hold against rotation relies primarily upon the grip afforded to a relatively narrow zone of the sheet material on which the nut is installed, this zone being gripped between a flange on one end of the sleeve abutting one face of the sheet about the hole in which the fastener is installed, and the expanded sleeve on the opposite side of the sheet. If the grip is inadequate the nut can rotate on the sheet.
It is also to be recognized that the illustrations appearing in patent specifications are necessarily greatly enlarged to make the features of construction plain. But in real life a typical nut may be of the order of 5 mm diameter at its maximum, and the annular zone provided by the flange is typically much less than 1 mm in axial dimension, i.e. wide. Consequently the actual surface available for gripping the sheet is minute.
Moreover, the possible taper and back taper may be exaggerated in the drawings of the said European patent for the same kind of reasons. And the actual resistance to rotation of the nut in the sleeve, other than what is due to the frictional effect may well be due to relative eccentricity again of the order of much less than 1 mm. Again then the actual angular hold is dependent upon minute differences in radial dimensions.
Consequently, it is desirable to provide improved means for both the anti-rotational hold of the nut relative to the sleeve and also of the sleeve relative to the installation.